The magnetic amplifier post regulator is a popular way to regulate the auxiliary outputs of a switched-mode power supply (SMPS). The SMPS couples power from a primary circuit to a main output circuit and an auxiliary output circuit through a transformer. A power width modulator and a power switch transistor control the transfer of power from the primary circuit. Therefore, the SMPS produces a main output voltage V.sub.OM and an auxiliary output voltage V.sub.OS. Magnetic amplifiers provide a simple, efficient, and reliable way of providing precise voltage regulation of independent outputs of a multiple output SMPS. The magnetic amplifier post regulator includes a magnetic amplifier, a reset transistor, and an error amplifier.
The error amplifier generates an error signal in response to sensing the auxiliary output circuit voltage, V.sub.OS. The error amplifier produces the error signal to control the blocking time of the magnetic amplifier by providing the appropriate reset of the magnetic amplifier core flux during T.sub.OFF of the power switch transistor. If the output voltage V.sub.OS is too high, the error amplifier turns the reset transistor on harder to supply more reset current into the magnetic amplifier in the reverse direction. Increasing the reset of the magnetic flux causes the blocking time of the magnetic amplifier to increase. This causes V.sub.OS to decrease. Similarly, if V.sub.OS is too low, the error amplifier causes the reset current to decrease, which in turn causes V.sub.OS to increase. This current reset technique then resets the magnetic amplifier core to a level that yields the correct reset time for the desired V.sub.OS.
The magnetic amplifier core may also be reset using a voltage reset technique. In the voltage reset technique, voltage clamping is applied at the output side of the magnetic amplifier. The current reset technique is more popular because of its ease of implementation and because it exhibits lower phase lag at high frequencies than does the voltage reset technique. Regardless of which reset technique the SMPS employs, the error amplifier is unable to control the reset of the magnetic amplifier core until its voltage rails, V.sub.CC and V.sub.EE, are established and the error amplifier output can slew sufficiently to gain control. A simple and cost-effective way to generate V.sub.CC and V.sub.EE is to use linear post regulators on additional auxiliary windings of the transformer. Unfortunately, it takes a finite time for these voltages to reach steady-state after applying input voltage V.sub.IN. Furthermore, the error amplifier slew rate is limited by the bandwidth necessary to yield adequate stability margins over all operating conditions.
In the meantime, the magnetic amplifier saturates within the first or second cycle of operation of the power switch transistor. The magnetic amplifier remains saturated until the error amplifier is able to provide reset current to the magnetic amplifier. This, unfortunately, causes overshoot of output voltage V.sub.OS.
There is a need, therefore, for a method and system for preventing output voltage overshoot and for controlling V.sub.OS until the error amplifier of the magnetic amplifier post regulator assumes control of the magnetic amplifier.
There is a need for a method and system that protects power supply loads against overvoltage conditions during start-up of an SMPS which employs magnetic amplifier post regulators to provide regulated power to the loads.
There is a need for a method and system for providing a well behaved increase in the auxiliary output voltages that a magnetic amplifier post regulator controls during start-up.
There is a further need for a general improvement in the overall start-up characteristics of the magnetic amplifier in a magnetic amplifier post regulator for a multiple output power supply.